An Iceland Enigma – The Thórsmörk Ignimbrite

Today we welcome a guest post by volcanologist Dave McGarvie. Dave is a senior lecturer at the Open University and studies volcanoes in Iceland and Chile. He can be found on Twitter under the username @subglacial.

One of the wonderful aspects of working as a volcanologist is Iceland is that fascinating new puzzles and their bigger cousins (enigmas) keep appearing. Sometimes even a fresh look at what seems to have been fully ‘sorted’ decades ago can – with the benefit of newer knowledge/understanding – result in enigmas re-emerging.

Welcome to the Thórsmörk Ignimbrite.

Authors Note:

What is an Ignimbrite? An ignimbrite is simply the rock that forms from the deposition of one or more pyroclastic flows*. Something extra needs to happen to convert the loose aggregation of pyroclasts and rock fragments that comprise a pyroclastic flow deposit into a rock (ignimbrite). Fortunately, pyroclastic flows are surprising good at retaining much of their heat during transport, so once they come to a halt this heat can cause sintering and welding – and thus the formation of a rock (ignimbrite) that is much more resistant to erosion. Compaction of the pyroclastic flow deposit can also occur, and again this transforms the deposit into something more rock-like. There are a host of other post-emplacement processes that affect pyroclastic flow deposits, but we’ll leave these as they are less relevant to this particular article. In the field, ignimbrites have textures too variable to describe briefly here. Fortunately, the Thorsmörk Ignimbrite is a bit simpler and has an ash matrix (light or dark coloured) with a scattering of pale-coloured pumices and rock fragments. These reflect the components of the original pyroclastic flow – ash and pumice being fragmented magma, and rock fragments being chunks of older rocks that were fractured and pulverised during the eruption (e.g. vent walls). For simplicity, I’ll refer to the ignimbrite as either welded, part-welded, or unwelded. *Note that I have chosen to use the term ‘pyroclastic flow’ instead of the jargon term ‘Pyroclastic Density Current’ (PDC for short), as I consider PDC a cumbersome and partly-misleading term.

Introduction

I first looked at the Thórsmörk Ignimbrite in September 2013, and after a couple of days in the field followed by some literature reading, I realised that there were still many unanswered research questions about the Thórsmörk Ignimbrite. Here’s a few of them:
1. When was this pyroclastic deposit erupted?
2. What was the source?
3. Is there just one ignimbrite present, or are there more?
4. What was the environment like when it was deposited?
5. What processes affected the ignimbrite soon after it was deposited?
6. Why are there dark and pale phases – are they the same chemically but just different physically?
7. Why is there so much reworking, and why had previous authors not mentioned it?
8. What do fragments of welded ignimbrite within overlying unwelded ignimbrite tell us about time gaps etc?
9. What did the original complete PDC stratigraphy look like?
10. When did the processes that jumbled the ignimbrite occur – during deposition, during welding, or post-welding (or some combination of the three)?

The answers to 1 and 2 are being worked on as I write, by PhD student Jonathan Moles (http://www.open.ac.uk/people/jdm539) The provisional and exciting answer to 2 appears to be Torfajökull to the NNE, but that’s a story for another time….

Figure 1

Figure 1 shows the location of the various volcanoes in southern Iceland along with exposures of the Thórsmörk Ignimbrite (in green).

Nobody is currently doing serious and comprehensive studies on the Thórsmörk Ignimbrite, so many of these questions will remain unanswered for the foreseeable future.

So, the aim of this blog article is just to give you some insight into what I’ve gleaned from a few days of fieldwork, and I’ll mainly address points 4-6 above. As this is recce-level work a nice robust model can’t be provided (it would be too speculative for my liking), but I hope that you will get a bit of insight into what volcanologists do in the field, complete with uncertainties, challenges etc.

Finally, another reason for this field-based article with lots of images is that this area is very accessible, and so if you – dear reader – wished to have a look for yourself, it is straightforward to do so. I’ll be happy to provide details of specific locations. Now, onto the nitty gritty.

Variations and themes

Figure 2

The first thing that struck me about the Thórsmörk Ignimbrite was its variability. From the logs in papers published back in the 1980s (see list at the end) I was expecting simple horizontal or gently dipping boundaries between different flow/cooling units that could be traced for hundreds of metres. Instead, it’s a mess, with different varieties of ignimbrite jumbled together on the scale of tens of metres. For example, look at Figure 2, which is c.300 long. Having looked at outcrops that appeared in published papers with Jonathan Moles and ignimbrite expert Becky Williams (@volcanologist on Twitter) we could not reconcile the complexities that were in front of our eyes with the simple logs in the published papers. This was very odd.

Several simple observations can be made from Figure 2: the base is not exposed; the top is exposed and is capped with sediments (which reassuringly contain clasts (i.e. chunks) of the Thórsmörk Ignimbrite – so the sediments are younger); there are pale and dark varieties; welding is variable; there is reworking of unwelded deposits. Yes, it really is a mess. The burning research question is – is this mess a mess related to original variable deposition of the PDCs, or is the mess due to later processes that moved separate ignimbrite domains in a way that brought them together?

So, let’s try and find some simpler exposures that might help us shed light on this variability. This is a common survival tactic when faced with a difficult field problem – go and look for simpler examples to understand first, then come back to the complex stuff.

Figure 3

Figure 3 (courtesy of Jonathan Moles) shows some of his mapping of southern Tindfjallajökull, where the Thórsmörk Ignimbrite occurs within a series of sedimentary units (tills) which – due to matrix and clast characteristics – we interpret as being glacial in origin. These units thin towards the margins of an ancient basin located in SE Tindfjallajökull, and in Figure 3 they can be seen thinning up to the right (towards the basin margin) with the ignimbrite sandwiched between tills. But is the complete ignimbrite preserved here?

Figure 4

No. Figure 4 shows that only one variety of ignimbrite is found – a part-welded dark phase. And although it looks simple from a distance, in detail it’s not as there are pods of other ignimbrite varieties around, such as the welded pale variety, and some unwelded ash. Yes, it’s still a puzzle.

But this area did demonstrate an interesting and previously-unknown feature of the local eruptive environment at the time the ignimbrite was being deposited – that a basin was filling with diamict (glacial sediments), and that this continued after the ignimbrite was deposited. The only difference between the diamicts below and above the ignimbrite is that the ones above contain clasts of ignimbrite.

Figure 5

A further look at ignimbrite exposures in this basin uncovered further complexity, including larger exposures of welded pale ignimbrite, further (rare) exposures of ash at the base, and that as well as being sandwiched within diamicts, the ignimbrite is also sitting directly on top of old basement rocks (pre-Tindfjallajökull), as shown in Figure 5. The latter merely indicates that the pre-ignimbrite topography was irregular, and that diamict was deposited irregularly.

Figure 6

Interesting insights into past environments that can be made from fieldwork in this area, is what happened after the ignimbrite was deposited. In many locations, subglacial basalts overlie the diamicts sitting on top of the ignimbrite (Figure 6). Logically, ignimbrite and diamict deposition would have required little or no ice to be present, but for subglacial basalts to be confined the ice must have been much thicker. So, it’s reasonable to conclude that a period of sustained cooling occurred after deposition of the ignimbrite. This points to a major environmental change after the eruption that formed the ignimbrite. See overlying subglacial basalts on Figure 6, and on Figures 3 and 4. And on Figure 7 (to come).

Figure 7

What else happened after the ignimbrite was erupted?

As well as the evidence for substantial subglacial basalt eruptions in SE Tindfjallajökull after diamicts had been deposited on top of the ignimbrite (as the basalts lie on top of the diamicts – see Figures 3, 4, 6, and 7), there is evidence in other locations that prior to subglacial basalt eruptions there was an episode of ‘reworking’ of the ignimbrite, involving especially the unwelded parts but also the welded part. In Figure 2 the label ‘reworked pale’ appears, and this reworked pale appears in many locations. Let’s look at a great exposure in Steinsholtsdalur, to the NE of Eyjafjallajökull.

In this location, there is the most spectacular and largest exposure of reworked Thórsmörk Ignimbrite that I have seen – but as I’ve not been everywhere, bigger and better ones may exist. At this exposure, reworked pale and unwelded pale ignimbrite occurs, and both are truncated by a subglacial basalt. The subglacial basalt is connected to small dykes that have intruded reworked ignimbrite, and in places the dykes have sintered (baked) the unwelded ignimbrite. Figure 7 shows Hugh Tuffen (@htuffen) and Lancaster University Masters student Alastair Hodgetts collecting sintered ignimbrite samples for lab analysis.

Figure 8

This is a marvellous exposure, and one worth spending days trying to decipher. For example, Figure 8 shows some detail of the reworked pale, with pumice-rich lenses and small faults affecting the finer-grained domains, and Figure 9 shows uppermost reworked pale cut by overlying subglacial basaltic volcaniclastics. There’s a great story to be uncovered.

Figure 9

Figure 10 reveals another puzzle about the reworked pale ignimbrite. Here, Becky Williams (@volcanologist) is near the base of the reworked pale, and a distinct shallow bedding can be seen. However, the bedding is dipping gently towards the east, which might imply deposition from a source to the west. Which is a puzzle, as there are no source volcanoes to the west that are known to erupt such compositions. Hekla’s magmas, for example, have quite different geochemical fingerprints.

Figure 10

I’ll take this opportunity to mention another puzzle: I’ve seen lots of reworked pale ignimbrite, but so far, I’ve not seen an exposure of reworked dark unwelded ignimbrite.

Figure 11

Moving on, there is ample evidence that the ignimbrite was eroded after being deposited. This is not surprising, given that we’ve seen evidence of thicker ice accumulating after ignimbrite emplacement (remember the overlying subglacial basalts). Like the ice currently forming Iceland’s ice caps and valley glaciers, this thicker ice would have been temperate (i.e. wet-based) ice, which is erosive. Much rarer polythermal (cold-based) ice, is much less erosive. A good example of erosion of a welded pale ignimbrite is seen in Figures 11 and 12, which are two views of the same exposure. This exposure also illustrates another notable feature that we see everywhere – the welded ignimbrite is always highly fractured, and not in a regular manner (e.g. columnar fractures). After welding, some unknown process has produced a series of irregular brittle fractures. This exposure permits the following time sequence to be suggested: ignimbrite emplacement, welding, erosion, then sediment deposition, then further erosion (as the sediments are themselves eroded). Interestingly, erosion of the welded ignimbrite occurred before the sediments were deposited, implying a time gap of unknown duration. And a final point – we don’t know what might be missing due to erosion, so the sequence above may be incomplete.

Figure 12

Ignimbrite Varieties

In this brief section, I’ll simply provide a few images of the different varieties of ignimbrite, and point out some of the puzzles to be solved. For me, the biggest and most crucial puzzle is the relationship between dark and pale ignimbrites – are they from the same eruption/source? Are they the same chemical composition, with the different colours related to a physical process (e.g. vesicle sizes/abundance)?

Figure 13

Figure 13 shows part-welded dark ignimbrite, whilst Figure 14 shows a foliated (or sheared) version of the same rock. There is considerable variation in lithics (non-juvenile clasts for the pedants) and in juvenile components (such as pumices), and Figure 15 (60 cm in length) shows a part-welded dark ignimbrite with abundant pumices and lithics, and note that some pale pumices contain lithics.

Figure 14

Figure 15

Figure 16 shows another oddity – a ‘spotted’ ignimbrite containing a mix of pale and dark domains. These are rare and only occur in small pods, and can be either welded or part-welded. Lab analysis would be needed to establish if there are two distinct (geochemical) components. Figure 17 shows a collection of welded to part-welded samples to illustrate just how variable this ignimbrite is.

Figure 16

Figure 17

Finally, we’ve found a few locations where pale and dark varieties are in close proximity. Figure 18 shows part-welded dark and pale ignimbrites in contact, with clearly different pumice contents (i.e. the dark is relatively poor in the white pumice clasts). This image is on top of a small cliff, and so the dark appears to be on top of the pale: if normal stratigraphic rules operate, the dark ignimbrite is younger. Interestingly, the layer immediately above the dark ignimbrite is rich in accretionary lapilli (Figure 19), and was the only good example we found. There are likely to be other exposures.

Figure 18

Figure 19

New evidence overturns an old model

I’m going to describe one bit of evidence that argues against Tindfjallajökull being the source of the Thórsmörk Ignimbrite. (There are other corroborating strands of evidence, but I’ll leave Jonathan Moles to tell the full story.)

Figure 20

The thickest exposure of the ignimbrite (c.200 m) logged in Karl Jørgensen’s 1980 classic paper was underneath a prominent rhyolite lava called Hestur on the lower SE flanks of Tindfjallajökull (Figure 20). This is almost 3 times thicker than any other measured thickness of the ignimbrite, so was I both curious and suspicious.

Figure 21

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To access the pyroclastic pile requires crossing a fast-flowing knee-thigh deep glacial river (Figure 21), which swells in the afternoon as ice melting increases, which in turn increases adrenalin. The first exposures encountered near the stream lacked the essential components of the ignimbrite (e.g. the expected matrix, pumices, lithics, phenocrysts, etc), and I assumed that I’d encounter the true ignimbrite higher up. But no, all the way up to the lava base the lithology was the same – an impersistently bedded pumice-ash breccia containing occasional fresh obsidian clasts. Apart from a completely different componentry to the Thórsmörk Ignimbrite, this pyroclastic deposit was intruded by lava lobes displaying pervasive small columnar joints (Figure 22), something that I’ve never seen at any true Thórsmörk Ignimbrite locality, and is not reported in the literature. The contact between the pyroclastic pile and the overlying lava is seen in Figure 23, and is often oxidised, which usually indicates near-simultaneous eruption between such rocks (see Figure 20).

Figure 22

Figure 23

So, this is not the Thórsmörk Ignimbrite, and once you remove this abnormally high thickness from the pattern, the argument that Tindfjallajökull is the source weakens dramatically. But what is this pyroclastic deposit? I had the advantage of seeing similar pyroclastic deposits at the Mount Rainier and Öraefajökull volcanoes, where such deposits are found underneath rhyolite and dacite lavas that have flowed along ridges flanked by ice (valley glaciers). These pyroclastic deposits are thicker where there are sudden drops in the ridge, which allows relatively volatile-rich lava in the core of the lava to vesiculate. I’ve not yet published the Öraefajökull work, but the classic paper on ridge-bounded lavas in glacial settings at Mount Rainier is worth a read if you are interested – see Lescinsky & Sisson reference at the end.

Endnote

There’s much more to the Thórsmörk Ignimbrite than I’ve described here, and it remains something of an enigma – hence the title.

What you are getting in this article is an example of fieldwork ‘in the raw’ complete with uncertainties and puzzles. Published papers never reveal the process of reaching the final interpretations and conclusions as there is simply no space (always at a premium). Instead, in published papers you get a more polished version that is designed to be convincing.

You will note a relative lack of interpretation and speculation in this article. This is deliberate as I prefer much more corroborating evidence before suggesting interpretations in public, and we’re not at that stage yet. Of course, I have my ideas of what happened and how the different components are linked, but I’ll leave these in my notebooks for the moment.

So, I hope I’ve given you a bit of insight into a fascinating and accessible enigma. And one in which if you have the interest, you can go and have a look at yourself, as there are daily buses into Thórsmörk throughout the summer and a short walk will take you to some great exposures.

Supplementary Information – background

Prior to 2013, some of the key aspects of the Thórsmörk Ignimbrite within published peer-reviewed papers were:
1. The largest late Pleistocene ignimbrite in Iceland. At least 6 km3 in volume, which equates to 4 km3 DRE (dry rock equivalent = magma).
2. Up to 200 m thick, comprising multiple flow units.
3. Varies from unwelded to welded.
4. Irregularly exposed in valley bottoms in the Thórsmörk area, plus on the lower south-facing flanks of Tindfjallajökull volcano to the north.
5. Depositional environment involved little or no ice.
6. Geochemistry indicates that it’s a rhyolite. (For those who are interested, it’s a comendite, which is a mildly-peralkaline rhyolite.)
7. Tindfjallajökull was the source.
8. This eruption may have created the caldera at Tindfjallajökull.
9. Linked with North Atlantic Ash Zone II (NAAZII) which is a major marine marker horizon dated to c.53-58 ka which is roughly the middle of the last glacial period.
10. Meltout of ice-rafted debris was responsible. Therefore, there was sea ice to the south of Iceland.
11. One of the few ash layers visible in Greenland ice cores. Therefore, the eruption had to have an explosive and far-travelled atmospheric phase in addition to the terrain-hugging PDCs that formed the ignimbrite.
12. The ignimbrite itself has never been convincingly dated.

Subsequent work, largely by PhD student Jonathan Moles (The Open University) has shown:
• Point 2. Incorrect. Removing an incorrectly identified exposure reduces the maximum true thickness to c.70 m.
• Point 7. Tindfjallajökull is not the source.
• Point 8. The so-called caldera at Tindfjallajökull may not exist – no clear field evidence for downfaulting was found, despite good exposure.

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117 thoughts on “An Iceland Enigma – The Thórsmörk Ignimbrite”

Dave, I hope you will continue to contribute to VolcanoCafe with more “fieldwork in the raw” articles. Seeing the varieties of ignimbrites in photos with your explanations was a treat, and I look forward to the end-results of your research of this area.

Thanks Spike – good to hear that you enjoyed it. I’ll see how this one goes, as I have several ‘fieldwork in the raw’ stories I could tell. And perhaps one leading to a published paper could be good – showing the progression of science 🙂

rescued from the pit of akismet, welcome Dave! All further posts will publish without issue. /Tam

Thanks for the post Dave. It’s really interesting to see the detective work, or rather day to day activities and thought processes, of a Geologist in the field. Looking forward to seeing the results when published … if not sooner 😀

Unfortunately, this work isn’t in a mature enough state to be published – no journal would touch it.

A lot more fieldwork is needed to provide a proper framework. I’ll do a bit of solo work next year, but it really needs at least a couple of trained Masters students working on it. Would be a lovely project, but then I am biased!

Ignoring basalt and considering only evolved magmas (rhyolite-dacite), best guess is one such eruption every 40-80 thousand years. It has to be a guess as we lack the ‘correct’ isotopes to accurately date large numbers of Icelandic rocks younger than about 500 thousand years old.

Icelandic rocks are typically low in K (Potassium), and hence low in the radiogenic Ar (Argon) derived from radioactive decay of K that is used in either Ar-Ar or K-Ar dating. The failures outweigh the successes, and there is a large unpublished dataset of failed attempts….

Unfortunately, magnetic polarity is unhelpful in dating young volcanic systems in Iceland, as it cannot not distinguish between separate eruptions – they all come out with normal polarity and so are between 0 years and 0.78 Ma in age 🙂

Marine NAAZII and the equivalent terrestrial ash layer in Greenland have been accurately dated to 53-58 ka. My PhD Jonathan Moles has been analysing these and Thorsmork Ignimbrite samples at the same time on the same equipment. The geochemical correlation looks sound. So yes, the proposed association still looks plausible.

There was a very brief reversal around 40,000 years ago. But that would only be helpful if there is a layer (above or below your ignimbrite) that formed within that brief period.

50,000 years ago would be one of the warmer period of the ice age. ‘Warmer’ is relative speaking, of course, but the ice cover may have been less than at the exreme depth of the ice age. And Iceland was never completely iced over, apparently. Anyway, the point is that a reducing ice depth may be the trigger of a major eruption, with perhaps the reducing weight allowing more melt to form.

Time markers in the Quaternary that are few and far between aren’t helpful, unfortunately. In contrast, the Holocene (postglacial) is so much better constrained because of the abundant tephra/ash layers of known age. Only in the elevated areas and desert areas of Iceland are these lacking, due to poor preservation. Foe best preservation of tephra, nothing beats soils with vegetation on top 🙂

The evidence for increased melt production following rapid ice mass removal from the Icelandic crust (i.e. deglaciation), is generally agreed, but some scientists argue that if you take out some of the ‘fliers’ (i.e. odd, large events), it’s not as big an effect as the advocates would have us believe.

Given that deglaciation has a strong effect on the mantle (as Iceland has a hot and low viscosity mantle, and therefore isostatic rebound is very rapid), the signatures of increased melt rate are seen in the geochemistry of basalts erupted in the fissure swarms.

We’ve looked for it, but we’ve seen no evidence of increased silicic magma production in Icelandic central volcanoes following deglaciation. (And the same appears to apply in Chile.) However, I do think there is some evidence for some extra basalt magma output at Icelandic central volcanoes in response to deglaciation, with Askja being a good example. We have a paper about to be submitted on this, which has good K-Ar dates of some of Askja’s Pleistocene basalts.

Excellent article on a fascinating deposit Dave; gives a very good taste of what fieldwork is like – walking up to an exposure of what at first looks like fubarite but slowly resolves and starts to make sense.

Before we should even attempt isopachs, we need logs of all exposures to gain a first-stage understanding of the deposit stratigraphy. We need to know how many pyroclastic flows there were, whether there were pauses (and erosion), and the lateral extent of each flow.

I estimate that I’ve visited only about one-third of the exposures. And these are the easier to access ones! Next year I plan to visit some far-flung exposures that will involve a few nights of wild camping.

Looks like a significant eruption has started at Fernandina, Galapagos Islands, a VONA has been released and on the Mirova thermal anomaly site, a rare extreme (purple) anomaly has been detected, almost 20,000 MW!

I have found that the more straightforward ‘problems’ are quite popular. And the more complex and/or hard-to-access areas are relatively untouched. Hence why I initiated the first serious investigations of subglacial rhyolites – which had the delightful combination of remote areas and tough fieldwork. My two first PhD students working on these were Hugh TUffen and John Stevenson (@htuffen and @volcan01010). Both tough characters and excellent field volcanologists.

One question I have – when ignimbrites form in a glaciated setting, is it possible for them to weld and settle on top of an active glacier (assuming a certain level of thickness). If so, could this possibly explain the irregularities?

Under this thought, as the ignimbrite cools, the glacier would still move, melt, and behave in an active manner, moving the rock above it as it goes. This would potentially lead to irregular deposition, transportation, and increased erosion of the above ignimbrite. Obviously, once the glacier would fully melt, that would result in the subsequent full deposition on top of the underlying rock layer.

Aha – a very insightful comment! This is the provisional model I am working on 🙂

The problem is, that if the ice is too thick, then the ignimbrite simply rides off on top of the glacier, becomes thoroughly dimembers, and is never seen again. There are known examples of this from central and south America.

The key point to reconcile is that whilst sea ice lay off Iceland’s south coast (as the ash from North Atlantic Ash Zone 2 was ice-rafted), just a few kilometres inland there was relatively little (or no) ice in the Thorsmork area. It’s a lovely puzzle.

I don’t expect simplicity in Iceland regarding why is Torfajokull caldera without a ice cap but Katla has a 600m thick cap. In a slightly warmer interglacial Katla would be ice-free, whisky in a slightly cooler interglacial, Torfajokull would probably show a ice cap as thick as Katla.

Reason for the difference: proximity to sea and higher rainfall, because the altitudes of such calderas are similar.

Another ice-free caldera: Askja and Krafla. Far less precipitation there, due to rainshadow.

All of the ice caps/sheets in Iceland only exist because they were once much bigger…which might seem a counter-intuitive thing to say. Put a different way, if you removed all of the ice in Iceland immediately, the ice caps/sheets would not grow back to anything near their current size. What a lot of fascinating volcanoes that would reveal 🙂

In stark contrast none of Hekla’s historic plinian eruptions seem to have produced pyroclastic flow deposits of any significance…….and the same goes for Askja 1875. This seems odd……one factor that has crossed my mind is that all of the pyroclastic flow producing VEI 4 and 5 eruptions that I listed all occurred in arc environments and involved phenocryst rich magma. Crystal rich magmas like this have a bulk water content much lower than that found in the actual melt. Loads of crystals burden the gas thrust part of the eruption column and do not release substantial water to help with transition to a buoyant column above the gas thrust.

In stark contrast, silicic magma in Iceland seem to erupt at near liquidus conditions and are crystal poor. Is it possible that this renders the “bulk” water content of the magma in these Icelandic eruptions higher than many crystal rich intermediate arc andesites and dacites, and tilts conditions in favor of buoyant, stable plinian eruption columns that are less likely to collapse?

Related to this point is the striking absence in the Icelandic Geological record of Peléan style domes with spines, whaleback features and talus fans, along with dome collapse produced block and ash flows. It seems that to produce this style of activity, magmas have to be so viscous that “melt viscosity” alone is almost never enough……..loads of crystals seem nessisary to increase the bulk or “whole rock viscosity” to the point were Peléan style activity can occur. For some reason this only ever seems to happen in arc settings. I am assuming this is somehow because of the extra water from the subducting slab, but I don’t know for sure.

A few other points…..local flow direction in ignimbrites sometimes follows local topography rather than spreading outward from the source……particularly at the base of pyroclastic flows. The lack of a pre ignimbrite pumice fall described here makes me think that the field outcrops described here may have been upwind of the erupting vents. Finally, it would be amazing to find tuffsite veins that might mark the vicinity of the vents that erupted this material, but that would probably be like looking for a needle in a haystack.

Thanks for your extensive comments. Interestingly, Icelandic rhyolites are more volatile rich than people first thought they were. Recent work by Jacqui Owens (supervised by Hugh Tuffen and myself) demonstrated this for water. The more alkaline rhyolites have unusually high halogen contents (F and Cl), which can have the effect of reducing viscosity and in turn reducing explosivity. This has not been fully explored 🙂

But arc systems definitely still dominate the explosive record because of their more persistently volatile-rich magmas.

As for Hekla and Askja, these are mafic-intermediate dominated systems with high magma throughput, and therefore lack the necessary time/stability to develop substantial volumes of silicic rock. There are old ignimbrites at Torfajokull, but these have never been studied thoroughly.

There are also old ignimbrites throughout the Tertiary (Palaeogene if you prefer), but I don’t think any exceed 10 km3 in volume (DRE). Again, a fairly unstudied aspect that could do with a modern and comprehensive study 🙂

PS. Glenn – the impersistent ash we occasionally find at the base of the Thorsmork Ignimbrite may represent an early fall (or surge) phase – and the lack of pumice may indidate phreatomagmatic fragmentation of the pumice into ash as the eruption breached overlying ice. We need to look at shard shapes in this ash to determine fragmentation mechanism(s).

We see similar fine-grained ash in the earliest phases of the 1362 AD Oraefajokull eruption, which transition up into massive pumice fall deposits (and pumice-rich pyroclastic flow deposits). But that’s another story – currently being written up in an MPhil thesis.

As for the vents, it is highly likely that after the explosive degassing that formed the ignimbrite, there was a waning phase of non-explosive degassing that formed rhyolite domes. If so, the vents are concealed 🙂
Cheers again, Dave

Does that mean that the dark/pale ignimbrite is based on clast size (rather than chemical composition)? could the dark ignimbrite be the initial layer that suffered phreatomagmatic fragmentation, with the pale ignimbrite being generated by the ongoing eruption after the ice was covered by the dark ignimbrite (and thus didn’t suffer from the phreatomagmatic interaction)?

You’ve hit on a key point – does the dark and pale matrix have similar or different geochemical compositions? I wish I knew for certain 🙂

Colour can be misleading: for example, there are golden basalt pumices (e.g. Grimsvotn 2011) and dark rhyolite pumices (Askja 1875).

The general pattern is that the dark ignimbrites sit on top of the pale ignimbrites, so dark appears to be younger. But in some places there is also pale ignimbrite on top of dark ignimbrite. It really needs a comprehensive mapping project to sort this out.

Feldspars only go pink when something (often circulating fluid) causes iron to be released from the crystal lattice and the Fe becomes oxidized. This is much more common in granites than in volcanic rocks.

The problem with Emma’s dataset is that it is not always clear exactly what she is analysing. This is why my own PhD student – Jonathan Moles – is constructing his own data set, some of which will included the pale and dark matrix. I’ll try and keep folks update 🙂

Hey Dave,. Great post, whilst I am not a geologist, I lived in Iceland for several years and I can say my opinion:

The terrain at Tindfjallajokull seems to show a crater rather than a caldera. Don’t think Tindfjallajokull is the responsible.

Research evidence for another Pleistocene great ring-faulted rhyolite eruption at Torfajokull. Something huge happened there, so it’s clearly one of the suspects. ( Update: just found out you did this research actually! )

Forget Hekla. Did not exist back then. Hekla is a very young volcano.

There is another source of ignimbrites and rhyolite in the region: Katla. There are Holocene lava flows from Katla in the NW outside of the glacier. A ignimbrite could have easily occured towards the west. Vedde ash eruption shows how much can Katla erupt. Often present in Greenland ice cores. Check those.

So these would be the two main suspects.

I would compare the Solheimar ignimbrite with this, and also with any Katla rhyolite.

Thanks for your views. Gives me an opportunity to add some more information 🙂

Tindfjallajokull. Icelanders are fond of putting calderas on their geological maps without having good/agreed criteria & the robust field evidence for doing so. When they are covered by ice and there is other evidence (e.g. faulting observed seismically, such as at Grimsvotn), perhaps this is acceptable. But you can form a caldera-like ring structure beneath ice entirely from construction – via effusion of subglacial silicic domes. So caution is needed. To give an extreme example, Kerlingarfjoll apparently has two calderas according to the maps, but there are no faults, and there is no evidence for subsidence. A geological map always contains a component of interpretation, but Kerlingarfjoll is wilder than speculation – imho 🙂

Torfajokull. Iceland’s best mapper (Kristjan Saemundsson) mapped Torfajokull, and acknowledges that the evidence for a massive caldera is circumstantial.

Katla*. Wrong composition to be the source of the Thorsmork Ignimbrite.

Solheimar Ignimbrite. Has been analysed in detail**. Comes from Katla. Very different composition to the Thorsmork Ignimbrite**.

All good wishes,
Dave

PS. If you’d like to read more about the geochemistry:
Christian Lacasse has done high-quality analyses of Katla rhyolites
Emma Nicholson did high-quality analyses of the Thorsmork and Solheimar Ignimbrites

Curiously, Icelandic geologists are relieved when they see fractures/features allowing heat to escape to the atmosphere, as this reduces the heat available for melting ice – and producing meltwater. For example, a huge collective sigh of relief was exhaled when the 1996 Gjalp ice cauldrons failed and the heat started escaping.

We are still in uncharted territory with Bardarbunga, so you are correct to question what might happen 🙂

Interesting article, I love rocks (see avatar 🙂 ).
Beautiful, how you try to reconstruct the history of the landscape developing!
A question about a term you use I poorly understand. What do you mean by “reworked”, it is something different to weathering I believe?

I am guilty of using terms that are less well known, and I should take more care to define them. My apologies 🙂

By ‘reworked’ I mean that a process has changed the deposit from its original appearance.

If you are observing a primary deposit with primary features, then you can make reasonable interpretations on the primary processes. However, when secondary processes have clearly affected a deposit, you have to throw away any notion of trying to understand primary processes.

The reworked deposits of the Thorsmork Ignimbrite involve unwelded primary deposits being broken up, transported, and then deposited somewhere else. (Probably by water or as ice-sediment slurries. We don’t know.) It is a sedimentary process, and forms a new sediment.

First I have to say that I enjoyed reading this article very much and hope there are many more of this kind to come! Thank you for your effort, Dave! I guess I will read it several more times to let it sink.

Apart from Galapagos there is other news: A new ice cauldron has formed in Vatnajökull – at a certain part of Bardabunga the ice seems to have come down to only around 100 meter in thickness. In the ice cauldron you can even see the stone ground beneath. Lets see if the molten ice will lead to a Jokulhaupt within the next few months.

Really depends. According to most of the models, South Florida is gonna catch the storm. Personally, I think it’s gonna have quite a bit of strength sapped out of it, but I’m not a weather expert. I just watch these things out of my own need to know what they might do. 1928 saw a similar scenario to what is developing here, the difference being that the official prognosticators are more prone to caution than back then.Fact; the storm has quite a bit of heft to it and is quite healthy at this moment.Rumination; The longer it stays in the high categories the better it’s flow system becomes. At some point in the ultra strong storms, they start to make their own steering currents, able to blow through other atmospheric features with ease. I’d like to say “don’t be there” but as I mentioned, I am far from being an expert. Likewise, I’m not gonna say stay either. Ultimately, it boils down to how you or your friend feels about the conditions and maximizing the odds for survival. I’m on the extreme end of the projected track if it doesn’t turn, but I am counting on the front dropping down through the SE US to add to the push for it to turn. The open ocean lump of water under it is about 44 feet. (The storm surge) But this is due to low barometric pressure and the strong venturi effect of the high winds. How that plays out as it transits the islands is hard to guess. I haven’t even found a projection from NHC about inundation yet, but they are the ones to refer to. Here is their latest discussion on it.

That is one seriously well-defined eye !
However it breaks , I hope everyone stays safe.
Dave, thanks for a very interesting article… A really good read.
As was the article on the Drakensberg… I was away and missed that one. When I was a kid, those mountains helped us escape the apartheid police, who were looking for my dad. In his student days he had spent many months working as a bearer to a geologist , tasked with studying rocks in Lesotho for the mining industry (late 50s ,early 60s),so he knew his way around the mountains there.
Mostly I saw the mountains at night, It’s beautiful !

Albert, you’re right to assume a long story. It would make for a great story if I could say that my dad was some kind of activist… but he isn’t.

He was a university lecturer by the time they took us all back to live in their homeland. He insisted on proper reasoning to back up otherwise unsupported assertions from his students. That was essentially the cause of the problem in the short term.If that was the whole story, I would be proud enough of him… but it isn’t… And I am extremely proud of him.
For another time, and maybe for another platform.
Oh… of some interest… We left his brother’s house following a telephone tip-off that the police were on their way with a banning order.The tip off came from a young journalist called Donald Woods, so I’m told.

I’m on the far end of Florida (nearly alabama) and the shelves here are already cleared of bottled water. According to the radio, the Florida attorney general already has 4000+ reports of price gouging. Thats a big no-no here. I’ve seen gas stations put out of business for pulling stunts like that. Legal fees stack up quickly when the state is after you. Something new that I haven’t seen before, allegedly they also made it illegal to fire someone if they are dealing with hurricane and miss work. Not sure about the details, like I said, it’s the first I’ve heard of it.

Late to the conversation Dave that is a great article-thank you.
In Oregon we have several major fires. But the Columbia Gorge
from Cascade locks through Multnomah falls is more or less gone.-with the fire wind and smoke.
The name of the fire is Eagle Creek-for the area that WAS one of the most beautiful places on the planet. Wife and I on our honeymoon trip went up the trail Waterfalls, ferns , Basalt formations. Now thanks to “Homo Stultis” -Teenagers with fireworks (illegal in Oregon ) that-in the presence of hikers
and, apparently their Parents, dropped firecrackers into the brush below them on the trail. I have no words. I am angry beyond belief. This was beyond stupid. As one who has spent years as an aerial fireifighter,
The gorge is a particularly nasty place to work. Steep narrow and turbulent.Add smoke and it gets dangerous real quick.
I hope and pray that murder is NOT added to the charges against the perps.
Behold the the Stupid and the results.:http://www.oregonlive.com/
Read all the updates and the backstory..

One other thing that part of the gorge is Basalt over laying
a sandstone formation called the Troutdale formation
it slides . and subsides, withe vegetation gone and winter coming
we are not done with this thing yet…

1) Go to Iceland and Bardarbunga
2) Left: Choose Your Data set(s) –> Enable ‘Sentinel-2’
3) Bottom right: Click “next” to ‘Acquired on 2017-07-25 at 08:55:54’ or also 1st August 2017
Those are sunny days where the cauldrons were visible.

Now to spaceweather news: a CME erupted a couple of days ago and it will trigger G2 or G3 geomagnetic storms above northern Europe and America, so a great time to see northern lights tonight, in some parts of the world. In Europe, they might be seen from parts of the UK and Denmark.
But even more interestingly, today a X9 flare occurred, the strongest of the decade, and its resulting CME will trigger impressive northern lights this weekend. Northern lights might reach as far south as France and Germany, in Europe.

Wow…. flashback. In 2004, Hurricane Ivan ripped through Pensacola. At the time, it was only a Cat 3, having declined in strength from it’s previous Cat 5 level. The one thing that this Sint Maarten video captures quite well, is the thundering bass of the winds. If you ever wanted to know what one sounds like, this is it. For me, the spooky bit was not knowing what was going on outside. I did take a peek out of my garage windows to investigate an odd light. What I saw was a tractor trailer cab turning around in my drive way. Evidently he was on the way to the electric power station up the road and the route was blocked. I have a fairly wide driveway and all of the vehicles were parked snug up against the garage door. I had even put up K type shoring to brace the middle of the door because of the expected winds. One of the benefits of having been through shipboard damage control training. The door held, but my Bronco was eaten by a tree.

Word of advice for any Floridians planning on riding this thing out. 1st, not a really wise idea. Yeah, I rode out Ivan but it was only a Cat-3 and I got lucky. 2nd, make sure you have a chainsaw with gas and oil stored away where you can get to it It might make it easier to find your way out afterwards, and last but not least. A sack of potatoes can go a long way towards helping you survive until aid arrives. Something to cook them on will be of great value. And keep lots of potable water around. For flushing the toilet, fill your tub with water and use a bucket to refill the basin as needed. For hurricane season, I make sure that I have at least 3 x 5 gallon water cooler jugs (and a hand pump purpose made for them). You can go without food far longer than you can without water. And, as you may know, Florida can get stupid hot. Stay hydrated.

I’m on the extreme opposite end of Florida from Miami, we are already having shortages of bottled water here in the stores. Some service stations are running out of gasoline, but that’s more from Harvey’s damage in Texas than anything else.

As for the chainsaw, MAKE SURE THAT YOU LEARN HOW TO SAFELY OPERATE IT. Even with that, do remember that debris from fallen trees does not act like a standing living tree. Limbs and branches are going to be in haphazard stress configurations and won’t necessarily respond to being cut the way you would usually expect. I had a fair amount of experience operating a chainsaw yet it still took me three days to cut my way to my truck. My caution was in trying to keep the rest of the tree from rolling the rest of the way over into my house. I already knew the truck was a write off, but the house was undamaged. It made sense to go slow and be careful. What really helped was that I had a (mostly grown) grand-kid available to act as a pack-mule in hauling debris.

Now, something that might come in handy. I took a 4 foot section of nylon line and put an eye in each end. One eye was large enough for me to slip a gloved hand into, the other was just large enough to pass the rope through as a chinch noose. Lay a few limbs or branches together and cinch the line around the base of them. You would be amazed at how much stuff you can drag along the ground on foot. By the time it was over with, I had a debris pile 150 feet long and 30 feet wide out by the roadway, and I could walk across my yard without tripping over anything.

It’s more than just the Peninsula at risk here, Geo. The most accurate wind model has 50 mph winds sustained all the way out to Destin/Ft. Walton for sure, and potentially Pensacola or beyond due to how enormous this storm is and how it might wobble 50 miles in either direction. Hurricane gusts for that area for certain, and for potentially as far inland as Atlanta.

There are Tertiary (Palaeogene) ignimbrites that are in better physical condition than the Thorsmork Ignimbrite, and these have seen millions of years of permafrost. But there maybe something peculiar about the Thorsmork ignimbrite and/or its environment that renders it more susceptible.

Whatever caused the fracturing happened after the ignimbrite has welded and fully cooled, as all fractures are brittle (i.e. no sign of ductile failure). I suspect that the substrate ‘moved’ slightly and the chief culprit is thin ice 🙂

From the above video; “It’s just a bunch of CGI” (referring to the magma drizzling into the sea) Funny that. I have gotten a similar reaction to molten glass drizzled into a bucket of water. (Yes, I am a member of Homo Stultus, I did stupid stuff as a kid) If it were CGI (which it’s not), they did a really good job of making it react the same way.

Volcanoes are not real. Yeah…. I flew once over Holuhraun, and the scenario was so outlandish that months later, years later, I still question myself. Did I just see a 30km long lava river, or was is just a crazy dream?

Unfortunately my photos showed me it was very real.
I still remember the weird smells and bonfire-like warmth. Ahahahah!

It was quite deep and off-shore. It won’t have affected any volcanoes, unless they were already fully primed to explode. No worries. San Andreas is also ‘safe’, i.e. quite capable of rupturing by itself but not accepting help from far south. A big quake can change the stress on nearby faults but this is not nearby! Anyway, wasn’t there a wall to stop that?

Does anyone here have any experience with Academia.edu? I listed myself with them as having an interest in volcanoes to get access to some academic papers on the topic, as my knowledge is much thinner than my interest. Now suddenly I find I am listed by Academia.edu as the author of a scientific paper on volcano hazards. Not only has my knowledge expanded exponentially via telepathic osmosis while I slept, I am informed that my new found expertise has been cited by other academics in their work. I have a couple of followers with Icelandic sounding names and someone wants me to link up with faculty members of the Geosciences department at the University of Sydney.

I have written a book on small business that had a few print runs, but I have never written papers on anything remotely connected with volcanoes, volcano hazards, geochemistry or petrology; other than flaunting my general ignorance on volcano cafe. I have heard of fake scientific journals acceptng and even producing fake scientific papers, but I never thought my name would be attached to one of them, If anyone here has come across the paper I am alleged to have written, I’d love to see it.I assume whatever meaningless jargon it is probably full of must be impressive.

The Chiapas earthquake apparently triggered a widely seen and video’d display of earthquake lights.
Here’s a Twitter feed from Metsul Weather, which has several video’s of the activity in the proximity of the quake (time-wise). If I didn’t know better, it just look like a regular lightning display, but apparently convection wasn’t happening at that time. A well-regarded climate researcher who hosts a California-based weather blog and works out of UCLA mentions that there are several geologists (unofficial I assume) who are giving these video’s credence. If so, it’s the first time I’ve seen video “proof” of earthquake lights.https://twitter.com/metsul

I’ve studied lightning a lot (and I mean: a lot), and these discharges are certainly ground to cloud discharges, but not focused on single lightning strikes. These “earthquake lights” look very stretched as if released over half a mile to several miles. (caution: some videos show local electric power blow outs: not to be confused). GL has the right of it. Rock stress static discharge of some kind.

It would be fascinating to stand in an area where they are being released. I doubt they are strong enough to do any harm, but in low cloud conditions, the plasma discharges must look amazing.

It’s good to see some decent video capture of the phenomenon. (If that is what it really is!)

Looking at the videos again, I think actually the great majority of these lights (if not all) are due to power blow-outs in the city. Impressive though they are, 90% of them have a local source. Oh well, disappointing.

… and Suburban Propane is into hoarding mode. None of their filling sites are operating. My guess is they are waiting for the prices to go up due to their artificially induced shortage. Sneaky way of bypassing the gouging laws.

I wonder if they realize that they just permanently lost my business? I am probably not alone in my opinion. Oh I’m sure they have some weak excuse, but I don’t have time for their line of bull$@$!. It’s not the first time I’ve seen them play this game.

I suspect that this is a very human way of responding a threat: finding crazy and odd ideas.
One example: I walk into the jungle and see a tiger. I can´t escape, the tiger is about to kill me. I quickly start praying to God, or try something else equally crazy, because in face of the impossibility of running or fighting the tiger, there might be just a solution that one has not foreseen. There is always a small probability that I can guess an unexpected solution or explanation.

Our brain does assumptions all the way. We do it every minute when we meet other people, and in everyday life situations. Assumptions are just that. It´s a way of iterating outside of the box, in order to find solutions. But some of those can get crazy.

– – – –

I would rather explain the 2017 hurricane season as akin to the 2005 hurricane season. It´s all to do with a few factors: 1) how much convective activity occurs in West Africa and near Cape Verde; 2) the steering eastwards winds triggered by both the Azores high and the Bermudas high, 3) unusual ocean warmth. If the 3 factors converge, then we can witness a very active hurricane season.

2017 saw record warmth not just in continental land, but especially in oceans worldwide. This helps a string of hurricanes that would normally stay around category 2 or 3, jump quickly to cat 4 or 5.

The timing of the MJO and Hurricane season might have something to do with it.

My favorite “almost a moon-bat” theory, is that when a drought of some sort is broken, nature tends to go overboard in fixing it. 12 years without a major hurricane making landfall, and then we get 2017.

On a more “AKK!” note, Port Everglades in Ft Lauderdale is the receiving point for most automotive fuel for South Florida. Their site has this to say; “The Port is currently closed to inbound ships.” Yeah, I was pretty miffed at the local propane company in an earlier comment, but the Port is under the safety guidelines of the USCG. The USCG is focused more on keeping people from getting dead. (They are quite good at it too… if you pay attention to what they say.) *yeah, I’m am ex Naval squid and I just said something complimentary about the USCG squids. I don’t care, they are highly skilled professionals and deserve the accolades. They risk thier lives to save people in conditions that only a crazy person would willingly go into. Me, I’d probably just throw you a line or a flotation device. They have the cajones to go after you. (and the equipment to minimize the hazard of also becoming a casualty) “Rescue Swimmers” are about the closest USN equivalent. That is if we can keep from killing them off in training. That caused a major rework of safety procedures throughout the Navy. Even to the point that I had to stop a class due to a student zapping themselves with static electricity on a dry winters day. That whole static electricity event was unbelievable, but my job was to err on the side of caution, even though I knew what had happened.

“The U.S. Coast Guard Captain of the Port has notified Port Everglades that Hurricane Port Condition ZULU has been set for Hurricane Irma for 8PM today, September 8, with gale force winds expected within 12 hours. Port Everglades will be closed to waterside and landside activity, with the exception of fuel trucks, until further notice unless specifically approved by the Coast Guard.”

Meanwhile, one of our BONEHEADED politicians plans to ride out the storm in South Florida. This idiot is just gonna get people killed. Why evacuate if Mr. Self Aggrandizement is sticking around? Anyone dying because they thought it was less of a threat due to his actions is on his head. Karma will get you buddy. Count on it. Everybody pays the piper, eventually.

Caveat: I’ve got room to gripe. I’m a Floridian. This reflects my own opinion and does not represent the views of the staff of Volcano Cafe. Technically, I should yank it under the “be nice” clause, but this wingnut can cause untold misery and harm by his self interest and trying to make a name for himself. Yeah, he’s in West Miami and should be clear of the storm surge, but if this storm breaches the levee of Lake Okeechobee like the 1926 storm did, he’s in one of the possible flow paths. (unlikely based on the potential rain estimate and the speed of the storm) but he’s set himself up for disaster, and others may consider him an example to follow when they should be getting out of the path. Already some of the late departures are having problems getting out of there.